1. Field of the Invention
Embodiments of the invention relate to methods for minimally invasive surgery and devices useful in such methods. More particularly, methods and devices described herein permit improved access within a body cavity to perform a surgical procedure, for example ablation and/or coagulation of cardiac tissue during minimally invasive surgical access to the heart. The diaphragmatic access described provides direct visualization of anatomic structures within the thoracic cavity such as the posterior left atrium, the posterior side of pulmonary veins, or any other such anatomic structure. In some variations, accessing the thoracic cavity in this manner facilitates manipulation of a coagulation probe to reliably create transmural, curvilinear lesions capable of preventing the propagation of wavelets that initiate and sustain atrial fibrillation, atrial flutter, ventricular tachycardia, or other arrhythmia substrate.
2. Description of the Related Art
Currently, procedures that provide access to the thoracic body cavity involve incisions through the chest wall. For example, such procedures include median sternotomy, thoracotomy, thoracostomy, or mini-sternotomy. Typically, these surgical techniques require deflation or retraction of the lungs to access the heart and/or other organs within the thoracic space.
A median sternotomy provides the most exposure for the physician. In this procedure the surgeon creates a midline incision through the sternum that cuts along the bone separating it into two sections. With a median sternotomy, although the heart can be lifted and manipulated by hand, the posterior surface of the heart or other organs is still not readily visible unless the heart is significantly rotated or lifted. However, significant rotation or lifting of the heart may cause undesirable hemodynamic issues during beating heart procedures. After the procedure, the surgeon closes the median sternotomy with large diameter metal wires. The rejoined tissue must be held stable during the healing process, similar to a bone fracture that must remain immobile during rehabilitation. Any coughing or dramatic movement is extremely painful to the patient because the chest moves. Clearly, rehabilitation after the medial sternotomy requires a significant amount of time.
Thoracotomy techniques involve creating large (or small with minithoracotomy) incisions between the ribs to gain access to the thoracic cavity. After the incision, the surgeon separates the ribs with a rib spreader to produce space for insertion of various instruments. The muscles that overlay the chest must be cut during the thoracotomy. Much of the pain during the rehabilitation process is due to the cutting of the muscles. A thoracotomy provides limited access and visualization to the heart unless endoscopes are used. Yet, even the use of endoscopes provides limited access to the posterior regions of the heart and lungs because these organs cannot be lifted or rotated easily.
Thoracostomy techniques use ports through the space created during the thoracotomy. The surgeon uses trocars (e.g. 6-10 mm) to access the thoracic cavity. Access to the anterior surface of the heart is generally sufficient with this technique. However, this technique does not provide ready access or visualization of posterior regions of the heart or other organs.
In subxyphoid techniques, the surgeon creates an incision below the xyphoid process but above the diaphragm. This technique is common for pericardiocentesis where blood is removed from the pericardial cavity during a pericardial effusion or tamponade. The diaphragm provides a barrier and hindrance to manipulating the heart or accessing the posterior heart surface during subxyphoid techniques. Accordingly, subxyphoid techniques are often limited to procedures that target the anterior or apical ventricular regions.
The conventional surgical techniques discussed do not provide the medical practitioner with sufficient visibility of anatomic structures within the thoracic cavity. For example, these procedures do not provide sufficient visibility for anatomic structures located along or adjacent to the posterior surface of the heart or lungs. In order to obtain such visibility, the patient must be placed on cardiopulmonary bypass support. Then the surgeon must create a large incision in the patient's chest through which the patient's heart and lungs can be lifted and/or rotated. Accordingly, surgical practitioners may be hesitant to treat tissues located along or adjacent to the posterior heart or lungs during less invasive procedures, given the inability to visually observe the target area. As such, minimally invasive cardiothoracic surgery has been limited to those anatomic structures located along the anterior surface of the heart.
Atrial fibrillation surgery is just one example of a surgical procedure that, while it relies on the surgical techniques discussed above, the procedure also suffers from shortcomings due to a lack of access to organs within the thoracic cavity. Atrial fibrillation surgery involving radiofrequency, DC, microwave, ultrasound, laser or other modes of thermal ablation of atrial tissue has a limitation where tissue contact throughout the length of the electrode(s) is/are not consistent. Such inconsistent electrode contact causes variability in the transmission of energy throughout the target length of ablated/coagulated tissue. This inconsistency also produces undesirable gaps of viable tissue that promote propagation of wavelets that sustain atrial fibrillation, or produce atrial flutter, atrial tachycardia, or other arrhythmia substrate. Target tissue regions that reside along the posterior surface of the heart is one factor that contributes to inconsistent electrode contact. As discussed above, conventional means of surgical access are not optimal to access the posterior surfaces.
Another factor that contributes to the inability of existing thermal ablation probes to create complete curvilinear, transmural lesions is the presence of convective cooling on the opposite surface of the atrium. This convective cooling produces a heat sink that decreases the maximum temperature at the surface thereby preventing the lesions from consistently extending transmural through the entire wall of the atrium. This phenomenon is especially significant during beating-heart therapies where the surgeon places the coagulation/ablation probe against the epicardial surface. However, because blood is still flowing along the endocardium, the blood removes heat. Heat convection produces a larger temperature gradient between tissue immediately under the probe electrodes along the epicardium and tissue at the endocardium. Increased tissue contact is capable of reversing this effect through compression of the tissue. This reduces the effective the wall thickness of the atria, ensuring consistent contact throughout the length of the electrode(s), and increasing the efficiency of thermal conduction from the epicardium to the endocardium creating a more consistent and reliable lesion.
Another deficiency of current approaches is the inability to direct the coagulation to precise regions of soft tissue while avoiding underlying or nearby tissue structures. For example, atrial fibrillation ablation may involve extending a lesion to the annulus near where the circumflex, right coronary artery and coronary sinus reside. In another example, atrial fibrillation involves ablating ventricular tachycardia substrates residing near the coronary arteries or coronary veins. In a third example, the esophagus resides along the posterior left atrium between the left and right pulmonary veins; unanticipated heating of the esophagus during atrial fibrillation treatment can produce esophageal fistulas which can be associated with high morbidity and mortality rates. Conventional approaches cannot selectively ablate the desired soft tissue structures while isolating other tissue structures that are intended to be preserved from targeted regions.
The improved methods and devices described herein offer improved access to tissue regions within the body, especially those organs in the thoracic cavity. Variations of these methods and devices address the above described deficiencies for atrial fibrillation and ventricular tachycardia ablation. In addition, the embodiments or variations of the embodiments may address similar deficiencies, which are apparent during other applications involving coagulation of a selected tissue region in a precise manner.